Radiant barriers can be anything that is very reflective (has a very low emissivity.) A good example of a radiant barrier is aluminum cooking foil since it reflects heat into or away from food while cooking. Since the 1930s (which is about when aluminum foil was invented) radiant barrier materials have proven themselves to be a potentially effective method of keeping unwanted heat out of buildings too. They are predominantly beneficial for buildings in cooling dominated climates.
Unlike the more common types of insulation (i.e., fiberglass, cellulose, etc. that trap pockets of a gas which in turn reduces heat conduction) radiant barriers reduce only radiant heat transfer. A single layer of reflective material, properly installed between the hotter roof deck and the attic floor, may reduce radiant heat transfer to the attic by about 95%.

Radiant barriers used in buildings to reduce cooling needs are usually installed directly under the roof to reduce heat gain from the sun. You may also see recommendations to apply them directly over any other attic floor insulation, however this strategy is NOT recommended since dust will quickly cover the reflective surface and reduce the insulating effect significantly.

Radiant barriers are also very effective for walls that get direct sunlight hitting them, and were building an effective roof overhang is not practical (for example walls facing west.) However, unless you plan on rebuilding the wall for another reason (or the cost of labor is nominal) this application may be too expensive to have a reasonable economic payback in retrofit situations.

It is also worth noting that research has proven that radiant barriers installed in heating dominated climates are not economic at all unless the material and labor costs are extremely low (almost free.) And, in some cases, radiant barriers have been seen to actually increase heating costs since the attic was kept cooler during sunny winter weather. This increased heat loss from the living space below during those sunny daytime hours.

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every day examples of radiant barriers......................................................over blackboard

How Radiant Barriers Work

Heat travels from a warm area to a cool area by a combination of conduction, convection, and radiation. Heat flows by conduction from a hotter material to a colder material when the two materials touch. Heat transfer by convection occurs when a liquid or gas is heated, becomes less dense, and rises. Radiant heat travels in a straight line away from the hot surface and heats anything solid as the wave of energy hits it.

When the sun heats a roof, it is primarily the sun’s radiant energy that makes the roof hot. A large portion of this heat travels by conduction through the roofing materials to the attic side of the roof. The hot roof material then radiates it’s gained heat energy into the cooler attic (some of the roof’s heat will radiate in other directions too.) A radiant barrier reduces the radiant heat transfer from the roof to the attic space.

Some radiant barrier installations work better than others. The key to understanding them is 1) the emissivity of the material’s surface; 2) the temperature of the reflective surface and the surface temperature of the source of the heat; 3) and the angle the heat hits the receiving surface.

Of these items, the angle the heat wave strikes the surface has the most influence on how well any shiny surface acts as a thermal insulator. All radiant barriers work best when heat hits the surface at a right angle (perpendicular.) From one brand of radiant barrier to another, how shiny the material is, and it’s emissivity, is so similar that it makes little difference as far as thermal performance. (Most products have emissivities between 0.03 to 0.05, which is the same as a reflectivity of 97% to 95%.) Also the greater the desired temperature difference between the sides of the radiant barrier material, the greater the benefits a radiant barrier can offer.
It may also be worth noting that a very glossy white paint is within 10% of as good reflector of heat as most of the common radiant barriers currently available in residential construction.

Since insulation of any type is intended to keep heat from moving in an undesired direction, the amount of thermal insulation already in the attic affects the benefits a radiant barrier may have in energy savings. For example, installing a radiant barrier in an attic that already has at least the DOE minimum recommendation of attic insulation for that climate will be less cost effective than an attic insulated to an inferior level. It is also necessary that an air space be on at least one side of the reflecting surface. If the material intended as a radiant barrier is "sandwiched" between two other materials without the air space the insulating effect from the foil surface is reduced to zero.

First determine if a radiant barrier is worthwhile for your climate. As mentioned before, they are intended for very sunny, hot climates to reduce cooling loads. Radiant barriers tend to offer little if any benefits in Northern heating dominated climates. They are also of minor value if your home’s roof is already heavily shaded by trees or has at least the minimum insulation recommended for your climate.

Most radiant barrier materials on the market today have about the same emissivity values. Therefore, you should consider other characteristics (strength, flammability, availability, and cost) before you buy.

Resistance to tearing is important especially for the "do-it-yourself" installation. A logical method for you to test its strength is to obtain a sample and try to tear it by hand. A barrier that tears easily may rip at fastening points and make installation difficult, if not impossible. The types that are least susceptible to tearing usually have a woven mesh laminated between two sheets of foil. Other types use a "bubble-pack" between the foil faces. This is similar to the plastic sheets typically used to cushion packages for shipping.

You should also check the flammability rating. Choose one that has a Uniform Building Code (UBC) Class I, or National Fire Protection Association (NFPA) Class A, flammability rating. If you are uncertain ask your local fire department or building inspectors office about suitability of the product you are considering.
Radiant barriers also direct heat back through the roofing materials and may raise shingle temperatures 1° to 10° F (17.2° to 12.2° C) but this has been proven to have no detrimental effects on roofing materials.

Since there is currently no standard method for equating how well a radiant barrier works when comparing it to other insulation types, many manufacturers use the term "equivalent R-value." Be wary of such claims since this has no scientific or legal meaning, and manufacturers’ claims often reflect optimum conditions and not necessarily climate conditions at your homesite.

There are a variety of installation options for radiant barriers: There are rolled foil types, radiant barrier paints, metal shingles, and roof sheathing that has a radiant barrier laminated to it. There is even aluminum foil "chips" mixed with blown-in cellulose insulation. All of these work in similar ways, but some types work a little better than others under specific circumstances.

When installing a rolled foil barrier, the easiest way to install it is during new construction. The installer typically drapes the radiant barrier, foil-face down between the roof rafters to minimize dust accumulation on the reflective faces (double-faced radiant barriers are available.) This is generally done just before the roof sheathing goes on if it’s not too windy, but it can also be done afterwards from inside the attic by stapling it to the bottom of the rafters.

When installing a foil-type barrier it is important to allow the material to "droop" between the attachment points to make at least a 1.0 inch (2.5 cm) air space between it and the bottom of the roof. This air space has mainly two functions: it creates an air channel for the soffit and ridge ventilation system to work more effectively; and acts as a second reflector since there are two shiny sides (one facing up/ one facing down.)

Some builders also try to attach the radiant barrier directly onto the roof sheathing prior to their installation on the roof rafters, but a more effective method is to simply buy foil-faced plywood sheathing instead. There are also metal roof shingles that have a reflective underside. If you needed roof shingles anyway, these are a practical option although the cost of the material is considerably higher than other types of radiant barriers.

As mentioned earlier, a radiant barrier on top of attic floor insulation is more susceptible to dust accumulation. This undesirable method may also trap moisture in the fiber insulation since, during cold weather, a radiant barrier on the cold side of the insulation acts as a vapor barrier in the wrong location. When warm moisture carrying house air leaks into the attic in the winter, it may condense on the underside of the barrier. Even a perforated radiant barrier can trap moisture in cold climates since the water can freeze in the small holes and seal them. Because of these hazards it is strongly recommend that you NOT apply radiant barriers directly on top of the attic floor insulation. Of course, installing it at all in a cold climate is not generally cost effective anyway.

During the summer, an attic radiant barrier, combined with existing R-19 attic insulation, will usually result in a total cooling load savings of 2%-15%. Buildings with little to no attic insulation and more than the usual amount of attic ventilation typically provides the most dramatic energy savings from a radiant barrier. The hotter and sunnier the climate is, the more beneficial the radiant barrier installation becomes. The reduced heat gain may also allow you to install a smaller air conditioning system, which results in even more saved energy.

For buildings in heating dominated climates (and with poor insulation on the attic floor) it is generally far more cost effective to install more than the minimum recommendation of ordinary insulation rather than a radiant barrier. This is because attics are often vented to the outdoors and heat entering the attic in the winter (through the inferior floor insulation) simply leaves the building through the attic vents whether or not a radiant barrier is present.

Also, in cold climates air conditioning is usually a much lower priority than heating. Cold-climate homes also tend to have higher attic insulation levels when compared to more Southern climates. As mentioned before, large amounts of common attic insulation negates much of the usefulness of a radiant barrier even under favorable circumstances.

Two field tests, one in Minnesota and one in Canada, both found that a radiant barrier placed over R-19 attic floor insulation (which is less than half the DOE minimum recommendation for those climates,) found that the radiant barrier contributed to less than a 1% reduction in energy consumption for heating and cooling.

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